Typical examples of electrochemical reaction systems are solid oxide fuel cells (hereinafter “SOFC”) using a solid electrolyte having oxide ion conductivity. The basic structure of such SOFC is made of unit cells in which there are connected three layers of an air electrode/solid electrolyte /fuel electrode.
A fuel gas such as hydrogen, a hydrocarbon or the like is fed to the fuel electrode of the SOFC unit cell, while an oxidizing gas such as oxygen, air or the like is fed to the air electrode, whereupon there forms a difference between the oxygen partial pressure in the fuel electrode side and in the air electrode side. Oxygen, which is ionized at the air electrode, migrates towards the fuel electrode via the solid electrolyte. Upon reaching the fuel electrode, the oxide ions react with the fuel gas, releasing electrons in the process. When a load is connected to the fuel electrode and the air electrode, therefore, electrical energy can be extracted directly from the electrode chemical reaction.
The geometrical shape of such unit cells can be classified into flat-plate and tubular shapes. Among them, known tubular SOFC unit cells have a structure comprising an (inner) fuel electrode/solid electrolyte/(outer) air electrode, where the fuel gas flows inside the tube, and an (inner) air electrode/solid electrolyte/(outer) fuel electrode, where the oxidizing gas flows inside the tube.
Tubular SOFC unit cells widely used at present have tube diameters of about 20 mm and lengths of about 150 mm. SOFC power source devices, in which a plurality of such unit cells are integrated by way of interconnectors or collecting wires, are problematic on account of their extreme size and low output density per unit volume. There are also limits as regards size reduction in such devices, since integration operations are hard and complex on account of, for instance, handling difficulties (Non-patent document 1).
Other than SOFC, electrochemical reaction systems that have been proposed include, for instance, exhaust gas-cleaning electrochemical reactors and hydrogen production reactors. As is the case with the above SOFC, however, size reduction and higher-density integration of the electrochemical reaction unit cells are also difficult in such reactors, which underlies the need for developing novel integration structures that allow achieving higher efficiencies.
When the diameter of a tubular electrochemical reaction cell in a given unit volume is shrunk to 1/N, the surface area of the unit cell becomes 1/N, but the number of unit cells that can be integrated per unit volume increases by a factor of N2. Therefore, it is estimated that there can be obtained an electrochemical reaction cell stack having an N-fold total surface area. As described above, however, conventional electrochemical reaction cell stacks have limits as regards size reduction and higher-density integration. In the technical field in question, thus, there is an urgent need for developing novel technologies and products that allow achieving both size reduction and higher-density integration in electrochemical reaction unit cells.
Non-patent document 1: N. M. Sammes, Y. Du, and R. Bove, J. Power Source, 145, 428-434 (2005)